Cell environment is a critical determinant of the ability of receptor-ligand complexes to activate transcription. Factors that can define ERalpha transcriptional activity include coactivators and corepressors, the relative ability of receptors to interact with cell-specific factors, changes in signal transduction pathways, or alterations in the coactivators/corepressors utilized to regulate ERalpha activity. The studies outlined in this application will provide important information on coactivator and corepressor usage at various ERalpha target genes, and assess the role of cell signaling pathways as a contributor to ERalpha activation by ligands. Our planned studies are based on four key observations. First, coactivators contribute to the transcriptional activity of ERalpha liganded with 4HT in contexts where this selective estrogen receptor modulator (SERM) is a partial agonist. Second, SMRT and NCoR are important for the antagonistic activity of 4HT. Third, intracellular signal transduction pathways affect the transcriptional activity of ERalpha, coactivators and corepressors. Lastly, alterations in intracellular signaling pathways accompany changes in cellular responses to partial agonists/antagonists such as 4HT. These observations have lead to the hypothesis that the ability of the SERM, 4HT, to regulate ERalpha-dependent biological events is modulated by the ability of coactivators and corepressors to functionally interact with the ERalpha within a given context, and that this is regulated by the expression of these molecules and the influence of gene- specific and cell-specific factors that regulate their ability to bind to receptor. This hypothesis will be tested in the following specific aims: 1) Determine SRC family coactivators' contribution to the ability of E2 and 4HT to regulate ERalpha- dependent gene expression; 2) Determine the contribution of the corepressors, SMRT and NCoR, to the ability of E2 and 4HT to regulate ERalpha-dependent gene expression; 3) Determine whether cellular environments influence the ability of CBP, SRC family members, and the corepressors, SMRT and NCoR, to interact with ERalpha in the absence of hormone or in the presence of E2 or 4HT and 4) Determine whether changes in intracellular signaling between estrogen-dependent and tamoxifen resistant breast cancer cells affect the ability of these pathways to alter interactions between ERalpha and coactivators or corepressors, and ERalpha- dependent gene expression. Our studies will therefore provide mechanistic information on 4HT's ability to modulate ERalpha activity in a gene or cell specific manner.